The apparatus and method of the invention utilize a two-photon resonance-enhanced multiphoton ionization (REMPI) instrument for trace species analysis. The invention is directed to a method and apparatus for utilizing a continuous flow of a liquid sample to detect and to identify trace organic substances in the sample. As REMPI is fundamentally a gas phase method the invention combines REMPI with membrane introduction mass spectrometry (MIMS), whereby organic compounds are extracted into the gas phase from a polar solvent such as water. A significant feature of MIMS is the simultaneous introduction of all organic analytes into the mass spectrometer. In many MIMS applications, the mass spectrometer is a standard quadrupole instrument, although both ion traps and triple quadrupole devices have also been used. Most of the studies using MIMS utilize electron impact or chemical ionization. However, the application of conventional ionization methods such as electron impact can make analysis of complex mixtures more difficult due to extensive molecular fragmentation. Accordingly, the invention combines MIMS with REMPI as the laser photoionization method, the latter of which may be adjusted so as not to produce photofragmentation. The combination of MIMS and REMPI provides sensitive and rapid analysis without prior separation or sample preparation and without deconvolution of multiple mass peaks.
While many of the analytes of interest which pass through the membrane to the photoionization zone may be photoionized using REMPI, there remains in the liquid sample analytes which either do not pass through the membrane. In particular, there may be analytes which are retained within the liquid sample flowing past the membrane that remain in solution. As a further embodiment, the liquid sample, after contact with the membrane, may be introduced into a capillary inlet tube which directs the liquid sample as droplets to the photoionization zone at subatmospheric pressure. Analytes in these droplets may be photoionized by REMPI.
As a further embodiment, it is realized that not all of the analytes, particularly the analytes which are not permeable to the membrane, may be readily photoionized by REMPI. Accordingly, both a radiation source for performing REMPI and a second source of radiation for performing single photon ionization (SPI) are provided. The two sources of radiation are selectively directed to the photoionization zone by a system of reflecting surfaces so that radiation from either source may be selected.
As yet another embodiment of the invention, there is a third source of analytes from the liquid sample, that is, compounds that are adsorbed or absorbed onto and into the membrane, but which do not pass through the membrane at the sampling temperature. Subsequent to photoionization and mass spectrometrical analysis of the other analytes, the analytes adsorbed/absorbed onto or into the membrane may be released therefrom by applying heat to the membrane or by running a different solvent to the membrane. This latter process would require halting the continuous flow of sample to the membrane, so it is preferred that heat be applied. These analytes will then pass through the membrane into the photoionization zone where they may be analyzed by REMPI or single photon ionization, as appropriate.
The present method and apparatus are applicable for detecting and identifying organic compounds in water samples without interference from the bulk water solvent. Thus, water samples such as ultrapure water for semiconductor processing, ground water, surface water, biological fluids, and potable water may be analyzed in real time for the presence of volatile organic compounds (VOCs), such as benzene, toluene, and xylene; for explosives, nitro compounds, organic molecules containing halogen, inorganic compounds such as metal and heavy atoms, aromatic ketones, large biomolecules, and the like. Because of their short-lived excited states, such molecules often cannot be detected using conventional nanosecond pulse-duration laser ionization sources. Typical detection ranges for the method according to the present invention using either the membrane or capillary inlet systems are in the range of about 1 ppb to about 1 ppt and the range of about 1 ppb to about 1 ppm of analyte in a sample.
Since sample preparation is not required, location of the apparatus need not be confined to a laboratory. A compact and portable analytical unit for sensitive and selective detection, identification, and quantification of trace organic chemicals and toxic compounds in water is provided by the invention.